The synthesis of diamond by various methods is well known and well established. One such example is the synthesis of diamond under high temperature and high pressure (HPHT). There are two principal methods employed for HPHT synthesis of diamond and both are from solution. The one method is a temperature gradient method and the other is an allotropic change method. In the temperature gradient method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of the carbon source material and the growing crystal as a result of a temperature difference between the two. The carbon that is present in the higher temperature region migrates to the seed crystal, which is positioned in the lower temperature region, via a solvent/catalyst material that separates the source material and seed crystal. Such a temperature gradient method is described generally in U.S. Pat. No. 4,034,066.
Almost all diamond synthesised from solution contains nitrogen and is diamond of type Ib. When growing type IIa diamond material which has a nitrogen content generally lower than 1 ppm (parts per million), removal of nitrogen from the starting materials is necessary. This is typically achieved by using a nitrogen getter. The nitrogen getter or agent is added to the solvent/catalyst, which is typically a molten alloy of the transition metals cobalt, iron and nickel. This agent has the effect of preferentially sequestering the nitrogen in the metallic melt, either as a solute or as a precipitated nitride or carbo-nitride. Such agents are typically elements like titanium, zirconium and aluminium.
The applicant's pending South African patent application Nos. 2004/9975 and 2005/04019 teach that type IIa diamond substrates can be grown in which the crystalline perfection in the {100}, {113} and {115} growth sectors approaches that of a perfect extended diamond cubic lattice and where the extended defect density is minimized. Such material has many potential applications, including use as a substrate in a subsequent chemical vapour deposition (CVD) growth process.
However, there remain four key limitations with the diamond synthesis techniques as taught in the above pending applications:                a) Cost. Using getters to control the incorporation of nitrogen is well known to reduce the growth rate of HPHT material. Although this invention still requires the growth of HPHT IIa material, using this as substrate material in a chemical vapour deposition (CVD) diamond synthesis process means that one plate might be used several times over, i.e. following the CVD growth the substrate might be reclaimed and grown on again. In addition the CVD process might be compatible with higher linear growth rates as well as replicating additional diamond plates of high crystal perfection from the first HPHT plate.        b) Although the growth sectors of HPHT synthesis can be controlled by parameters such as temperature and pressure, there are always {111} growth sectors which have a higher concentration of extended defects.        c) It is very difficult to controllably dope HPHT material with dopants such as P, B, N, Li, Na, Al, Si, S, etc. which might be needed in some applications which, for example, use the electronic properties of diamond.        d) Producing multilayer samples, where within one diamond there might be two or more layers which differ in the point defect density, such that there is a sharp interface both in space and doping concentration, is very difficult using HPHT techniques.        
In addition, using HPHT synthesis methods it is very hard to eliminate impurities such as boron to the extent required in some applications that require material with the intrinsic properties of undoped diamond. Although to some extent these disadvantages can be overcome, the parameter space for growing high crystalline quality diamond makes it very difficult to control all these issues within a single high pressure-high temperature growth run.
Annealing is a method well known in the field of materials science to improve the crystal domain size and perfection of the domains generally. Usually annealing is performed at an elevated temperature with or without an increased pressure depending largely on what part of the diamond-graphite phase space the annealing is performed in. For example, U.S. Pat. No. 5,908,503 discusses the use of a high temperature furnacing stage, typically using temperatures of 1100° C. to 1600° C. and low pressures, using a non-oxidising atmosphere to improve the crystalline perfection of diamond. The non-oxidising atmosphere is a requirement to prevent the oxidation and loss of the diamond crystal during treatment.
Annealing of extended lattice defects in diamond requires diffusion of carbon atoms. The diamond lattice is a very tightly bonded lattice and diffusion is restricted except under certain conditions. Increasing temperature increases diffusion, but increasing pressure generally reduces it. In type Ib diamond the presence of significant levels of nitrogen in the diamond significantly enhances diffusion. Although some prior art (V. D. Antsygin's article in Optoelectronics Instrumentation and Data Processing (1998) No. 1 p9) have shown an improvement on the crystalline perfection in BARS grown type Ib diamond on annealing at 2100° C., the extended defects and annealing mechanism in type Ib and IIa are quite different and the prior art teaches that it is very difficult to remove structural defects in diamond containing a low nitrogen concentration.
‘BARS’ is the acronym for a Russian-developed type of apparatus used for applying high pressures and high temperatures that is also referred to as the ‘split-sphere’ technique by those skilled in the art.
Methods of depositing or growing material such as diamond on a substrate by chemical vapour deposition (CVD) are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F, Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen and for boron. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy wherein x and y can each be 1 to 10 or halocarbons CxHyHalz wherein x and z can each be 1 to 10 and y can be 0 to 10 and optionally one or more of the following: COx, wherein x can be 0.5 to 2, O2, H2 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled; for example hydrogen may be present as deuterium or tritium, and carbon may be present as 12C or 13C. Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF (radio frequency) energy, a flame, a hot filament or jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
The prior art teaches that dislocations in CVD diamond arise from three sources: continuing from dislocations in the surface upon which CVD growth takes place, nucleation of new dislocations at the interface, and by dislocation multiplication during growth, particularly where the dislocation density is already high.
CVD diamond may be produced on a variety of substrates. Depending on the nature of the substrate and details of the process chemistry, polycrystalline or single crystal CVD diamond (the object of this invention) may be produced. A need exists for single crystal diamond material having exceptional crystalline quality and controlled point defect concentrations. A further need exists for a method for creating such diamond.